The invention relates to a process for the automatic control of a plant for processing a flow of gas with a strongly fluctuating SO.sub.2 content, and is aimed at obtaining a flow of SO.sub.2 -containing gas such that SO.sub.2 is discharged at a steadier rate.
More particularly, a flow of gas containing SO.sub.2 may come from a combustion unit fired on sulfur-containing fuel, such as certain fuel oils or coal. It is undesirable for such gases to be discharged into the atmosphere without precautions being taken, and several different methods are known that are intended to prevent pollution of the atmosphere by this cause. This requires the removal of the SO.sub.2 from the gas and its conversion into a non-volatile substance.
One method of SO.sub.2 removal is the conversion of SO.sub.2 into elementary sulfur according to a Claus process, which may or may not be modified. In the Claus process, sulfur is formed according to the reaction 2H.sub.2 S+SO.sub.2 .fwdarw.3S+2H.sub.2 O. Obviously, the supply of H.sub.2 S and SO.sub.2 has to take place in the stoichiometrically required quantities, because otherwise the gases issuing from the Claus unit cannot yet be discharged. In case of a varying supply of SO.sub.2, the supply of H.sub.2 S must vary accordingly, or, when the supply of H.sub.2 S is constant, an additional and variable supply of SO.sub.2 must be applied, for which a separate source of SO.sub.2 must then be available. In both cases the varying SO.sub.2 content of the gas flow to be processed causes considerable operational problems.
Another method for the conversion of SO.sub.2 into a non-volatile compound is the conversion of the SO.sub.2 into H.sub.2 SO.sub.4. For this process a variable supply of SO.sub.2 is also very undesirable.
In general, the concentration of SO.sub.2 in flue gases or in other spent gases is lower than 5 percent by volume. This concentration is too low for direct treatment in a conversion unit to be applied. Moreover, the flue gases in most cases still contain oxygen, which causes an additional consumption of reducing agent in the Claus process unit. Therefore, it is important that use be made of an intermediate step in which the SO.sub.2 -containing gas is concentrated and at the same time the SO.sub.2 is separated from the oxygen.
In the Netherlands patent application No. 7,102,211, a method is described in which the SO.sub.2 is bound directly to a solid such as CuO. The solid acceptor material must be regenerated to enable it to be used repeatedly. This can be done with a reducing gas such as H.sub.2. As a result, SO.sub.2 will be released again and this SO.sub.2, in a concentration which on the average will be higher, can now be further processed together with steam and reducing gas. This gas contains hardly any oxygen. However, the SO.sub.2 content of this gas varies from almost zero at the beginning of the regeneration to a maximum value, after which it decreases to almost zero again at the end of the regeneration. The SO.sub.2 concentration in the regeneration gas is from about 5 percent to about 20 percent by volume, and is mostly about 10 percent by volume.
A gas in which the SO.sub.2 concentration varies to such a large extent is also obtained if the flue gases or other spent gases are treated with a liquid absorbent, which in turn is regenerated.
It is an object of the invention to provide a process by which the varying quantities of SO.sub.2 present in such gases can be controlled so that the concentrations of SO.sub.2 are more evenly distributed.